ktest: Reset saved min (force) configs for each test

The min configs are saved in a perl hash called force_configs, and this
hash is used to add configs to the .config file. But it was not being
reset between tests and a min config from a previous test would affect
the min config of the next test causing undesirable results.

Reset the force_config hash at the start of each test.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>